1. Field of the Invention
The present invention relates to a side light type spread illuminating apparatus, especially to a spread illuminating apparatus for use as a lighting means for a liquid crystal display device.
2. Description of the Related Art
A side light type spread illuminating apparatus, in which a primary light source is disposed at a side surface of a light conductor plate, is predominantly used as a lighting means for a liquid crystal display (LCD) device used in a mobile telephone, and like devices. Conventionally, the primary light source has been constituted by a cold cathode lamp. Currently, a point light source, such as a white light emitting diode (LED), is heavily used, which is easier to handle, enables easier downsizing and is more resistant to impact shock than the cold cathode lamp.
Such a spread illuminating apparatus using a point light source is expanding its application field, and the application is now directed not only to a small LCD device for use in a mobile telephone but also to a relatively large LCD device for use in, for example, a car navigation system. In order to sufficiently illuminate a large display area, various approaches have been attempted to efficiently utilize light emitted from the primary light source and also to increase the amount of light emitted.
One of such approaches is disclosed (refer to, for example, Japanese Patent Application Laid-Open No. 2003-215546: Paragraph [0027] and FIG. 2 therein), in which an LED is disposed tightly close to a light conductor plate so that light emitted from the LED can be efficiently introduced into the light conductor plate. FIG. 3 shows a conventional spread illuminating apparatus incorporating the aforementioned approach, in which a light inlet surface LP of a light conductor plate GLB makes contact with the light emitting portions of LEDs 1 and 2, and inward curvatures PJ1 and PJ2 are formed at one side LW2 of a frame-like molded case MLD so as to protrude toward the LEDs 1 and 2 thereby resiliently generating forces (F) to press the light conductor plate GLB against the LEDs 1 and 2, which ensures a close contact between the LEDs 1 and 2 and the light inlet surface LP of the light conductor plate GLB thus allowing lights emitted from the LEDs 1 and 2 to be effectively introduced into the light conductor plate GLB.
Another approach is to increase the amount of light emitted from a primary light source by increasing the number of LEDs disposed at a side of a light conductor plate, or by increasing the current applied to an LED so as to increase the light amount per LED. This approach, however, causes an increase in heat generation at the primary light source thus raising the ambient temperature.
Generally, the luminous efficiency of an LED is lowered in proportion to an increase in temperature, and also in the case of the spread illuminating apparatus of FIG. 3 in which the LED and the light inlet surface of the light conductor plate are in contact with each other, heat generated at the LED is transmitted directly to the light inlet surface of the light conductor plate thus increasingly allowing the light inlet surface to suffer damages, such as heat deformation. To deal with the heat generation problem with the LED, the LED and the light conductor plate are housed in a metallic chassis such that the LED makes contact with the chassis for enhancing the radiation performance (refer to, for example, Japanese Patent Application Laid-Open No. 2004-186004: Paragraphs [0035] to [0037] and FIG. 2 therein).
However, in the case of the spread illuminating apparatus of FIG. 3 in which the LED is in contact with the light inlet surface of the light conductor plate, it is difficult to completely eliminate the heat deformation problem only by arranging the LED to make contact with the metallic chassis for enhancing the radiation performance as described above. As for the prevention of the heat deformation, it is preferable that the LED should not make contact with the light conductor plate. On the other hand, when the LED is disposed at the light inlet surface of the light conductor plate with an air gap provided therebetween, variation in gap size causes variation in brightness and visual quality.